1. Field of the Invention
This invention relates to a control for an idle speed control valve which controls bypass flow of intake air in a period of idling.
2. Description of the Prior Art
In an idle speed control is provided a bypass path parallel to an intake path portion provided with a throttle valve. An idle speed control valve (hereinafter called "ISC valve") is provided in the bypass path and the opening of the ISC valve is controlled by control pulse signals. Assuming that 0% or 100% of the duty ratio of the control pulse signal is designed to fully close the ISC valve, conventional control system for an ISC valve sent the control signal having a duty ratio of 0% or 100% to the ISC valve when the ISC valve was to be fully closed. In actual control circuits, however, absolute 0% or 100% of duty ratio is not attained and pulses or gaps (low level voltage portion between adjacent pulses) with fine width took place and, as noise, rode on the signal line of a knock sensor there resulted adverse effects on the knock control of the ignition timing or supercharging performance was degraded in the supercharging period without completely closing the ISC valve.